rtt calculation handles time go backwards

webrtc/modules/rtp_rtcp/source/rtcp_receiver.h

 /*
  *  Copyright (c) 2012 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 59 matching lines @@
 
     // get rtt
     int32_t RTT(uint32_t remoteSSRC,
                 int64_t* RTT,
                 int64_t* avgRTT,
                 int64_t* minRTT,
                 int64_t* maxRTT) const;
 
     int32_t SenderInfoReceived(RTCPSenderInfo* senderInfo) const;
 
+    void SetRtcpXrRrtrStatus(bool enable);
     bool GetAndResetXrRrRtt(int64_t* rtt_ms);
 
     // get statistics
     int32_t StatisticsReceived(
         std::vector<RTCPReportBlock>* receiveBlocks) const;
 
     // Returns true if we haven't received an RTCP RR for several RTCP
     // intervals, but only triggers true once.
     bool RtcpRrTimeout(int64_t rtcp_interval_ms);
 
@@ skipping 195 matching lines @@
   // when did we receive the last send report
   uint32_t _lastReceivedSRNTPsecs;
   uint32_t _lastReceivedSRNTPfrac;
 
   // Received XR receive time report.
   RtcpReceiveTimeInfo _remoteXRReceiveTimeInfo;
   // Time when the report was received.
   uint32_t _lastReceivedXRNTPsecs;
   uint32_t _lastReceivedXRNTPfrac;
   // Estimated rtt, zero when there is no valid estimate.
+  bool xr_rrtr_status_ GUARDED_BY(_criticalSectionRTCPReceiver);
   int64_t xr_rr_rtt_ms_;
 
   // Received report blocks.
   ReportBlockMap _receivedReportBlockMap
       GUARDED_BY(_criticalSectionRTCPReceiver);
   ReceivedInfoMap _receivedInfoMap;
   std::map<uint32_t, RTCPUtility::RTCPCnameInformation*> _receivedCnameMap;
 
   // The last time we received an RTCP RR.
   int64_t _lastReceivedRrMs;
 
   // The time we last received an RTCP RR telling we have successfully
   // delivered RTP packet to the remote side.
   int64_t _lastIncreasedSequenceNumberMs;
 
   RtcpStatisticsCallback* stats_callback_ GUARDED_BY(_criticalSectionFeedbacks);
 
   RtcpPacketTypeCounterObserver* const packet_type_counter_observer_;
   RtcpPacketTypeCounter packet_type_counter_;
 
   RTCPUtility::NackStats nack_stats_;
 
   size_t num_skipped_packets_;
   int64_t last_skipped_packets_warning_;
 };
 }  // namespace webrtc
 #endif // WEBRTC_MODULES_RTP_RTCP_SOURCE_RTCP_RECEIVER_H_